It’s new.

MERS was first seen in Saudi Arabia in 2012. Since then it has spread to 16 countries, most in the Middle East but also in Europe and Asia and now including the United States. It’s a coronavirus, a distant relative of the SARS — severe acute respiratory syndrome virus — that infected 8,000 people around the world and killed nearly 800 before it was stopped.

It’s deadly.

WHO reports more than 250 confirmed cases and 93 deaths since the virus was identified in 2012. But Saudi Arabia alone has reported 371 cases, with 107 deaths — that’s a roughly one-third death rate.

Tests of people who have been in contact with patients show some apparently get infected and do not get sick, so it’s hard to know what the actual death rate is. Most who die have been either elderly or had another illness, such as diabetes or kidney failure. But Dr. Anne Schuchat of the U.S. Centers for Disease Control and preventions says that’s still bad. “For respiratory viruses that have symptoms, this is a very severe clinical presentation,” she told reporters.

It doesn’t spread easily.

Health officials have closely studied the known patients, and the people who become infected usually have been in close and prolonged contact. There’s been no documented spread on an airplane, for instance. A patient in France who died infected a person who shared his hospital room, and family clusters have been reported from Saudi Arabia.

"We believe that if good infection control precautions are used during healthcare the risk to healthcare workers is also exceedingly low," says Dr. William Schaffner of Vanderbilt University, an infectious diseases expert.

There’s no treatment.

There’s no vaccine against MERS, although some groups are working on one, and antiviral drugs don’t appear to be of much use against it, either. The CDC has told U.S. hospitals to take strict precautions if someone shows up with symptoms and has recently traveled to the Middle East. Health care workers in contact with such patients should wear special face masks, gloves and gowns and follow other protocols to protect themselves and other patients.

Schuchat says the patient in Indiana is getting supplemental oxygen but is not on a ventilator, although severe cases have needed respiratory support.

No one knows where it came from.

The latest research suggests camels, but many patients have had no known contact with camels. Camel meat or milk might be a source, and the virus can live on surfaces and potentially could spread when people touch an infected surface. SARS was eventually traced to an animal called a civet, often sold in Asian markets as food.

Some health officials say it's possible MERS has been circulating but no one knew what it was because there wasn't a test for it. Many respiratory diseases are never diagnosed.